Meibomian glands are specialized sebaceous glands in the eyelids that are responsible for producing meibum. Meibum is an oily substance that forms the outermost layer of the tear film slowing its evaporation. In humans, there are approximately 50 meibomian glands present in the upper eyelid, and approximately 25 meibomian glands in the lower eyelid.
Meibomian gland dysfunction (MGD) is generally described as a chronic, diffuse abnormality of the meibomian glands. MGD may result in an alteration of the tear film. MGD is thought to be a significant cause of dry eye disease throughout the world (Nichols et al., Invest. Ophthalmol. Vis. Sci. 52:1922-1929, 2011).